Pulse Output Relay Equivalents


  • Are the Pulse WattNode outputs like Form A or Form C relays?
They are like a Form A relay contact but are polarity sensitive because they use a transistor.
  • Are the Pulse WattNode outputs dry contact?
No, but they are generally compatible.


The Pulse WattNode uses optoisolators to generate the output pulses. These approximate a relay contact closure and are compatible with almost all devices that expect a relay, but there are some differences.

  • Optoisolators can only switch a limited current, 8mA for the WattNode meter outputs.
  • When fully turned on (conducting), there will be a small voltage drop across the optoisolator, generally ranging from 10 millivolts (mV) up to a volt or more if the current is high (8 mA).
  • Optoisolators are polarity sensitive, so they cannot be used to switch AC voltages directly and the applied DC voltage must be wired with the correct polarity.

Specifications (from WattNode Pulse Manual)

  • Isolation: 5000 Vac RMS
  • Breakdown Voltage (collector–emitter): 60 V (exceeding this may destroy the outputs)
  • Maximum Reverse Voltage (emitter-collector): 5 V (exceeding this may destroy the outputs)
  • Maximum Leakage (Off) Current (collector–emitter): 100 nA
  • Recommended Load Current (collector–emitter): 1 µA (microamp) to 5 mA (milliamp)
  • Maximum Load (collector–emitter) Current: ~8 mA
  • Output Rise Time (microseconds): approximately Rpullup / 100, where Rpullup is the pullup resistor value (in ohms) and the pull-up voltage is 5 volts. Rise time is defined as the time for the output voltage to rise from 20% to 80% of the pull-up voltage.
  • Output Fall Time: approximately 2-3 microseconds with a 5 V pull-up voltage.
  • Saturation Voltage vs. Load Current: this is the typical voltage (at room temperature) measured between the COM terminal and P1, P2, or P3 when the optoisolator is on (conducting). Ideally, this voltage would be zero, but instead, it varies with the load current.

WattNode Pulse meter optoisolator dropout voltage graph

Solid-State Relays

For a closer approximation to an actual relay, see Option SSR. Option SSR substitutes optically isolated solid-state relays in place of the optoisolators, and can switch much higher currents. They also have a low on-resistance, so they more closely approximate

Relay Types

  • Form A: Single pole, single throw (SPST), normally open. The WattNode meter outputs are like Form A relay contacts.
  • Form C: Single pole, double throw (SPDT), break the connection with one throw before making contact with the other (break-before-make). The WattNode meter outputs are NOT Form C compatible.
  • Dry Contact: Relays that do not make or break significant current, so that arcing is not a concern. The WattNode meter outputs are generally compatible with devices that work with dry contact relays.

See Also

Keywords: opto-isolator, FormA, FormC